1. Field of the Invention
The present invention relates to a zoom lens of the type generally called the standard zoom lens which has a zoom range ranging from wide angle to telephoto with a standard focal length range therebetween.
2. Description of the Prior Art
As wide angle zoom lenses there have conventionally been used such zoom lenses consisting of two lens groups with a diverging lens group at the front of the zoom. The advantage of this type of zoom lens is that there is the possibility of substantial reduction in diameter of the front lens and also of better correction of aberrations at the wide angle end of the zoom range. However, this type of lens system is not suitable for the design of a zoom lens having a larger zoom ratio than 3:1 while covering even the telephoto side (the ratio of the maximum focal length to the minimum is called the zoom ratio).
If the zoom lens of the above-mentioned type is so made as to have a larger zoom ratio than 3:1, then aberrations such as spherical aberration and coma produced by the front diverging lens group are no longer completely corrected but remain as remaining aberrations which have adverse effect on the performance of the zoom lens in particular on the telephoto side of the zoom range.
Another basic form of zoom lens is of the type having a converging lens group as the front lens and consisting of three or four lens groups. In order to realize the desired small diameter of the front lens employing this type of the zoom lens it is required for each lens group of the zoom lens to have a stronger refractive power and also to be moved only a short distance for zooming. Even when these requirements are satisfied, the variation of aberrations with zooming becomes uncontrollably large. Therefore, in this case, it is difficult to reduce distortion, astigmatism and spherical aberration to acceptable level throughout the available zoom range. This difficulty becomes large in particular when it is desired to realize a compact zoom lens employing the basic form of zoom lens comprising three lens groups of positive, negative and positive or employing the basic form comprising four lens groups of positive, negative, positive and positive. By increasing the refractive power of each group to realize the compact design, the power of the negative lens group is rendered very strong and the above-mentioned difficulty in aberration control is markedly increased.
The difficulty is lessened when there is used such type of zoom lens comprising four lens groups of positive, negative, negative and positive. Since this type of zoom lens includes two positive lens groups and two negative lens groups among which the necessary power is apportioned, a compact zoom lens can be designed without sharing unreasonably large power to any particular one of the lens groups. In this case, therefore, it is possible to design the zoom lens more compact while moderating the difficulty in correcting aberrations.
Some standard zoom lenses employing the above basic form consisting of four lens groups of positive, negative, negative and positive have already been known and disclosed, for example, in Japanese Patent Application laid open No. sho-53-47,451 (the counterpart of which is U.S. Pat. No. 4,192,577) and Japanese Patent Application laid open No. Sho-56-48,607 (the counterpart of which is U.S. Pat. No. 4,390,248).
Although the share of power to each lens group has been reduced to some extent in these prior art lenses, they have still some problems to be solved. Firstly, the known zoom lenses still suffer from relatively large variation of aberrations with zooming. In particular the variation of astigmatism is large and that of distortion is markedly large. Secondly, the reduction in diameter of the front lens achieved by the prior art is relatively small. The zoom lenses proposed by the above-referred patents are not so compact to call them compact zoom lens. These drawbacks are particularly noticeable in the standard zoom lens having a zoom ratio large enough to cover the range of from wide angle side to telephoto side.